The present invention relates broadly to a photodiode detector apparatus, and in particular to a Gallium Phosphide pn junction detector apparatus.
A photodiode is a semiconductor two-terminal component with electrical characteristics that are light-sensitive. All semiconductor diodes are light-sensitive in some degree, unless enclosed in opaque packages, but only those designed specifically as such may be called photodiodes.
Most photodiodes consist of semiconductor Pn junctions housed in a container designed to collect and focus the ambient light close to the junction. They are normally biased in the reverse or blocking directions, therefore, the current is quite small in the dark. When they are illuminated, the current is proportional to the amount of light falling on the photodiode. Photodiodes may be used both to detect the presence of light and to measure light intensity.
The photoelectric behavior of semiconductors, such as germanium or tellurium, differs in a semiconducting emitter completely occupying a closed band of energies, which lies just below a so-called forbidden energy band. The electrons behave quite differently from those in metals. As a result, the photoelectric threshold energy is larger than the electronic work function W. Thus, a semiconductor exhibits a higher photoelectric threshold energy than a metal having the same work function. An example, the metal platinum and the semiconductor tellurium both have the same electronic work function about 4.8 ev. The photoelectric threshold of the platinum is equal to the work function, whereas that for the tellurium is quite higher.
A particularly interesting and important kind of photo-emitter is typified by cesium antimonide, Cs.sub.3 Sb. This material is a semiconductor having a forbidden energy band about 1.5 ev wide. The photoelectric threshold energy is only slightly higher than this. Electrons excited from the occupied energy band by incident photons cannot assume energies lying in the forbidden band, they must remain in conduction band. Thus, even the slower ones must retain energies only slightly less than that required for escape. The probability of photoemission is higher than for metals for semiconductors that have threshold energies greater than twice the width of the forbidden energy band. Ca.sub.3 Sb is sensitive over much of the visible range and can give very high yields in excess of 0.2 electron per incident photon. It is widely used in practical phototubes. The present invention achieves a greater quantum efficiency than heretofore obtainable at relatively small wavelengths.